Diesel engines are a target of increasing development activity by combustion engine and motor vehicle manufacturers because they offer the potential for lower emissions and increased fuel economy as compared to gasoline engines. Diesel particulate filters (DPFs) are being developed as components of diesel engine exhaust systems in order to control particulate exhaust emissions by physically trapping soot particles present in exhaust steam in their structure. Among the diesel particulate filters being developed are porous ceramic wall-flow filters, i.e., porous honeycomb monoliths end-plugged in a manner that forces exhaust gas flow through the porous ceramic walls, collecting any particulates present in the exhaust gas on or within the upstream walls of the structures.
Over time, the particulates collected by the filter increase pressure drop across the filters and thus exhaust gas back-pressure within the engine exhaust system. Therefore, once a predetermined soot loading condition is met, the filter is cleaned by a so-called “regeneration” cycle during which the temperature of the exhaust gases or filter are increased to a level sufficient to ignite and bum particulate soots. This regeneration cycle reduces the backpressure of the diesel particulate filter to approximately original levels.
The surfaces or interiors of the walls of these exhaust filters may support oxidations catalysts such as platinum (Pt), palladium (Pd), iron (Fe), strontium (Sr) or rare earth elements such as cerium (Ce), typically supported by high-surface-area washcoats, such catalysts acting to lower the temperatures required for soot combustion and filter regeneration. In flow-through ceramic catalyst supports used to treat gasoline engine exhaust gases, such catalysts promote the conversion of hydrocarbons and carbon monoxide in the exhaust gases to non-hazardous water vapor and carbon dioxide.
One preferred material for the manufacture of high temperature ceramic catalyst supports and filters is cordierite (Mg2Al4Si5O18), a refractory and low-thermal-expansion magnesium aluminum silicate offering high strength and good thermal shock resistance. Cordierite ceramics are typically manufactured by mixing raw batches comprising oxide sources such as talc and clay together with alumina and silica, binders such as methylcellulose, and lubricants such as sodium stearate to form plastic mixtures that can be extruded into green honeycomb shapes, dried, and fired to reaction-sinter the oxide materials into low-expansion ceramics.
Ceramic wall-flow filters made by the alternate channel plugging of these ceramic honeycombs can be extensively evaluated by diesel engine bench testing to evaluate catalytic performance, regeneration efficiency, filtration efficiency, pressure drop, and long-term durability. Such evaluations have demonstrated that soot loading distribution, flow distribution, catalyst distribution, and even the pore size distribution along the filter can directly influence the temperatures reached in honeycomb filters during the regeneration process.
To capture temperature changes arising in such filters during bench testing, arrays of thermocouples are inserted into the filters at various locations along the lengths and across the diameters of the filter structures. These thermocouples enable the precise determination of temperature levels and profiles along and across the filter as the regeneration cycles proceed. Instrumentation at these levels has confirmed that different locations within such filters reach different temperatures during regeneration, in some cases resulting in large temperature gradients within the filters and in others resulting in damage to the ceramic structure itself. Further, the temperatures and temperature gradients reached have been found to depend directly on the soot loadings present within the filters at the start of regeneration, and the manner in which the regeneration cycle is initiated and controlled by engine operating systems that can affect exhaust gas compositions and flow.
The maximum temperatures and temperature gradients reached during filter regeneration have been found to correlate directly with filter survivability and durability. Unfortunately, however, the extensive bench testing instrumentation used to determine peak filter temperatures and temperature gradients cannot be practically employed to measure or control the regeneration cycle in operating motor vehicles. Accordingly, there is no practical way of determining whether or when the design limits for long-term filter operation might have been exceeded during vehicle operation.